Implanted antenna and radio communications link

ABSTRACT

A medical implant comprising a housing and an antenna member carried by the housing. The antenna member is configured to be capacitively coupled to body tissue in which the housing is implanted. This forms, together with the impedance of the body tissue, part of a resonant circuit. A reference electrode carried by the housing as a return for the antenna member also forms a further part of the resonant circuit. Transceiver circuitry is also provided and is operable as at least one of a source and a load for the antenna member and forms yet a further part of the series resonant circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from Provisional PatentApplication Nos 2003907152 and 2003907153 filed on 30 Dec. 2003, thecontents of which are incorporated herein by reference.

BACKGROUND

Implantable devices in body tissue which operate at the Medical ImplantCommunications Service (MICS) frequency need to overcome the lossymedium created by body tissue.

Antennas operate most efficiently at a dimension which is a quarter of awavelength of signals transmitted or received by the antenna. However,using the MICS frequency range the optimum quarter wavelength wouldresult in an antenna having dimensions of approximately 0.2 m. Anantenna of this length would be exceedingly difficult to implant into apatient without significant surgical invasion and discomfort to thepatient.

Any discussion of documents, acts, materials, devices, articles or thelike which has been included in the present specification is solely forthe purpose of providing a context for the present invention. It is notto be taken as an admission that any or all of these matters form partof the prior art base or were common general knowledge in the fieldrelevant to the present invention as it existed before the priority dateof each claim of this application.

SUMMARY

Throughout this specification the word “comprise”, or variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated element, integer or step, or group of elements, integers orsteps, but not the exclusion of any other element, integer or step, orgroup of elements, integers or steps.

According to a first aspect of the invention, there is provided amedical implant comprising:

-   -   a housing;    -   an antenna member carried by the housing, the antenna member        being configured to be capacitively coupled to body tissue in        which the housing is implanted to form, together with the        impedance of the body tissue, part of a resonant circuit;    -   a reference electrode carried by the housing as a return for the        antenna member, the reference electrode forming a further part        of the resonant circuit; and    -   transceiver circuitry operable as at least one of a source and a        load for the antenna member and forming yet a further part of        the series resonant circuit.

According to a second aspect of the invention, there is provided amethod of transmitting a signal between a medical implant and anexternal unit arranged externally of a patient's body, the methodincluding the steps of:

-   -   feeding signals to an antenna member of the implant, the antenna        member being capacitively coupled to body tissue in which the        implant is arranged; and    -   using the capacitively coupled body tissue as an extension of        the antenna to transmit the signals between the implant and the        external unit.

In one embodiment, the resonant circuit can be a series resonantcircuit. The series resonant circuit may be completed by an impedancematching network which is arranged between the antenna member and thetransceiver circuitry. The impedance matching network may comprise aninductor and other components. The impedance matching network may beused to transform a total load impedance of the body tissue and theantenna member capacitance to a value optimised for maximum powertransfer between the transceiver circuitry and the body tissue.

The impedance matching network may further perform other optimisationand signal coupling/decoupling functions.

The impedance matching network may be electrically adjustable tocompensate for varied implant conditions.

In another form of the invention, the antenna member may be arrangedwithin the housing. The housing may therefore be a radio opaque housingto allow strong capacitive coupling through to the body tissue. Thehousing may be sufficiently thin to create a capacitance between theantenna member and the body tissue and may further be arranged to have ahigh dielectric co-efficient to increase the capacitance.

In yet another form of the invention, the antenna member may be arrangedexternally on the housing to be in direct contact with the body tissuewhen the housing is implanted. In this case, the impedance matchingnetwork may contain a low frequency block to isolate low frequencystimulation currents.

The reference electrode may be arranged externally of the housing to bein direct contact with the tissue. Instead, the reference electrode maybe capacitively isolated from the body tissue by being placed within thehousing. It will be appreciated that the reference electrode and theantenna member are capacitively coupled to each other.

The transceiver circuitry may comprise a radio frequency circuit whichallows communication in the MICS frequency band. The transceivercircuitry may contain a combination of a transmitter, a receiver, ademodulator, a detector and related signal processing circuitry.

Where the implant is used as a stimulation device, for example, afunctional electrical stimulation (FES) device, the housing may containa stimulation drive circuit module. The stimulation drive circuit modulemay communicate with the transceiver circuitry via a data link. The datalink may be a digital radio data and control link. The stimulation drivecircuit module may generate the required signals for stimulation, maymonitor the condition of an internal battery of the implant and may bemicroprocessor controlled.

The antenna member may be arranged to capacitively couple RF currentinto the body tissue. Thus, the antenna member may have a surface arealarge enough to effect capacitive coupling between the antenna memberand the body tissue. However, because the capacitance of the body tissueat the MICS frequencies is significantly smaller than that of theantenna/body tissue capacitance, the dominant capacitance is the bodytissue capacitance.

According to a third aspect of the invention, there is provided amedical implant which includes:

-   -   a housing;    -   control circuitry contained within the housing;    -   a plurality of electrodes each of which is connected via a lead        to the control circuitry; and    -   a frequency-dependent isolating module interposed between at        least one of the electrode leads and the control circuitry to        permit the at least one electrode and its lead to function as an        antenna for transmission of signals between the control        circuitry and an external unit arranged externally of a        patient's body.

According to a fourth aspect of the invention, there is provided amethod of transmitting signals between an implanted medical implant andan external unit arranged externally of a patient's body, the implantincluding a plurality of electrodes each of which is connected by anelectrode lead to control circuitry in a housing of the implant, themethod including the steps of:

-   -   generating signals to be transmitted to or from the implant;    -   electrically coupling at least one of the electrodes and its        associated lead to the control circuitry of the implant; and    -   using the at leat one electrode and its associated lead as an        antenna to effect transmission of signals between the implant        and the external unit.

As indicated above in the third and fourth aspects, the implant mayinclude a plurality of electrodes each of which is connected to thecontrol circuitry via a lead. Each lead may feed through the housing ofthe implant through a hermetically sealed feed through element.

The control circuitry of the third and fourth aspects may includetransceiver circuitry which feeds data, at a radio frequency, to, andwhich receives radio signals from, the external unit. The transceivercircuitry may also include an impedance matching network for theelectrode and lead functioning as the antenna.

The control circuitry may further include a stimulation circuit modulewhich contains the electronics for the predetermined stimulationapplication of the implant. The stimulation circuit module may bemicroprocessor controlled and may include a battery for operation of theimplant, associated electronics, measurement circuitry and, optionally,other radio transceivers.

The frequency-dependent isolating module of the third aspect by whichthe electrode and its associated lead are connected to the controlcircuitry may comprise, firstly, a radio frequency choke which passes alower frequency stimulation signal to enable the electrode to operate asa stimulation electrode, the radio frequency choke further blockinghigher, radio frequency signals. Further, the isolating module mayinclude a radio frequency coupling element which blocks the lowfrequency stimulation signal and couples the radio frequency signal toground.

Optionally, certain additional electrodes with their associated leadsmay be used as ground references for the antenna in addition to beingused as electrodes for stimulation purposes. These additional electrodesmay be coupled to the control circuitry via isolating modules as well.Hence, each of the additional electrodes may be connected to the controlcircuitry via a radio frequency choke which passes a low frequencystimulation signal but which blocks the radio frequency signal and aground radio frequency coupling which blocks the low frequencystimulation signal and couples the radio frequency signal to ground.

Each of the remaining electrodes with their associated electrode leadsmay be coupled to the control circuitry via radio frequency chokeswhich, once again, pass the lower frequency stimulation signals butblock the radio frequency signals.

The implant of the third and fourth aspects may further include a returnelectrode plate mounted on the housing. The electrode plate may bemounted externally on the housing to be in direct contact with bodytissue in which the implant is implanted. Instead, the electrode platemay be mounted within the housing to be isolated from direct contactwith the body tissue but which still allows capacitive coupling to thebody tissue.

The electrode used as the antenna may be radio frequency isolated fromthe body tissue by manufacturing a choke-inline with the antennaelectrode at the appropriate length. Instead, the antenna may have theelectrode capacitively coupled to the body tissue so that the bodytissue is used as an extension of the antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described by way of example with reference to theaccompanying drawings in which:

FIG. 1 shows a schematic representation of a medical implant, inaccordance with an embodiment of the invention;

FIG. 2 shows a schematic representation of current generated in bodytissue as a result of a capacitive coupling of the implant to the bodytissue;

FIG. 3 shows a simplified equivalent circuit of the representation ofFIG. 2; and

FIG. 4 shows a schematic representation of a medical implant, inaccordance with an alternative embodiment of the invention.

DETAILED DESCRIPTION

As shown in FIGS. 1-3, reference numeral 10 generally designates amedical implant, in accordance with one embodiment of the invention. Theimplant 10 includes a ceramic or radio opaque housing 12 to allow strongcapacitive (high value) coupling between an antenna member 14 arrangedwithin the housing 12 and body tissue, indicated generally at 16, inwhich the implant 10 is implanted.

The implant 10 includes a return electrode 18 which, in this embodimentof the invention, is arranged externally of the housing 12. Theelectrode 18, which is in the form of an electrode plate, provides aconnection to the body tissue 16 of the patient and acts as a returncurrent path for radio signals from the antenna member 14. The electrode18, can also be used as a return electrode for a stimulation signalgenerated by the implant 10.

Optionally, the return electrode 18 can be mounted within the housing 12to be capacitively coupled to the body tissue 16.

The antenna member 14 is in the form of a plate and is designed tocapacitively couple RF current into the body tissue 16. As illustrated,the antenna plate 14 is mounted under the surface of the housing 12. Theplate 14 has a surface area large enough to create a large capacitancebetween the antenna plate 14 and the body tissue 16. This capacitance islarge compared to the tissue capacitance of the body tissue 16 at theMICS frequency used for the implant 10. Thus, the total antenna load isdominated by the body tissue capacitance, allowing the body tissue toperform the major role in the antenna resonant circuit. Typically, thecapacitance of the antenna plate 14 to the body tissue 16 is an order ofmagnitude greater than the capacitance of the body tissue 16.

Because the antenna plate 14 is arranged within the housing 12, it isnot in physical contact with the body tissue. This has the advantagethat the antenna plate 14 can be made of any suitable conducting metalto optimise the radio frequency characteristics and need not be made ofbio-compatible metal. In addition, the antenna plate 14 could be a partof a printed circuit board 20 on which components of the implant 10 arearranged. It will be appreciated that, in an embodiment (not shown)where the antenna plate 14 is arranged externally of the housing 12, theantenna plate 14 needs to be of a suitable bio-compatible implantablemetal such as platinum, titanium, stainless steel, or the like.

The implant 10 includes transceiver circuitry 22. This circuitry 22 is aradio frequency circuit that allows communication on the MICS frequencyband. The circuitry 22 contains a combination of transmitter, receiver,demodulator, detector and signal processing. The antenna plate 14 isconnected to the circuitry 22 via an impedance matching network 24.Typically, the impedance matching network 24 is a passive inductor andcapacitor network.

The impedance matching network 24 is used to transform the total loadimpedance comprising the capacitance between the antenna plate 14 andthe body tissue 16 to a value optimised for maximum power transferbetween the transceiver circuit 22 and the body tissue 16, enabling thebody tissue to radiate the radio frequency signal.

The impedance matching network 24 also performs secondary functions andmay contain other circuit items to optimise voltages, optimise thecurrent, optimise the impedance, protect the transceiver circuit 22,effect power saving, effect other signal coupling or decoupling andfilter unwanted signals. The impedance matching network 24 is designedto have low radio frequency losses.

In the case where the antenna plate 14 is mounted externally of thehousing 12 in direct contact with the body tissue 16, the impedancematching network contains a low frequency block to isolate the lowfrequency stimulation tissue currents when the implant 10 is operable asa stimulation device. In this regard it is also to be noted that, wherethe implant 10 operates as a stimulation device, the return electrode 18could operate as a return path for stimulation currents.

The impedance matching network 24 is connected to the transceivercircuit 22 via a link 26. The link 26 couples RF power in bothdirections between the transceiver circuit 22 and the impedance matchingnetwork 24.

As indicated above, the implant 10 could operate as a stimulation devicesuch as, for example, a functional electrical stimulation (FES) device.Thus, the implant 10 includes a stimulation drive circuit module 28. Themodule 28 is microprocessor controlled and generates the requiredsignals for stimulation. The implant 10 is battery operated via aninternal battery 30. The module 28 monitors the condition of the battery30 via a connection 32.

The module 28 communicates with the transceiver circuitry 22 via a link34. The link 34 is a digital radio data and control link for effectingexchange of digital data and control signals between the module 28 andthe transceiver circuitry 22.

Finally, a connection 36 is provided for establishing electricalconnection between the transceiver circuitry 22 and the return electrode18. This connection 36 couples RF power in both directions between thetransceiver circuitry 22 and the return electrode 18. In use, data to befed from the implant 10 to an external control unit (not shown) arrangedexternally of the patient's body is transmitted from the transceivercircuitry 22 to the antenna plate 14. The antenna plate 14 is configuredto be capacitively coupled with the body tissue 16 such that radiofrequency currents, as represented schematically at 38 in FIG. 2 of thedrawings, are generated in the body tissue 16. This current isdistributed in the body tissue 16 and causes a radiation effect similarto an antenna point source in a lossy medium. In other words, the bodytissue 16 functions as part of the antenna of the implant 10 and effectsenergy transfer to the external unit. Hence, the need for a physicallylarge antenna 14 for the implant 10 is obviated.

In this regard it is to be noted that the implant 10 functions in theMICS frequency band which is in the range 402 MHz to 405 MHz. A quarterwavelength antenna for such a frequency band would have a physicaldimension of approximately 20 cm. With the use of the body tissue 16 aspart of the antenna structure, the antenna plate 14 need not have such alarge dimension.

Because the housing 12 is radio opaque, other electro-magnetic signalscan be transmitted to the implant 10, for example, an inductive coil forrecharging the battery 30 or to add additional current when highstimulation currents are required.

Referring to FIG. 3, it is to be noted that the antenna plate 14together with the impedance of the body tissue 16 forms part of a seriesresonant circuit to effect energy transfer into the body tissue 16.

The antenna plate 14 is represented in the series resonant circuit as acapacitor due to its mounting in the housing 12 and being capacitivelycoupled to the body tissue 16. Because the antenna plate 14 is mountedwithin the housing 12 it need not be of a biocompatible material and isa copper plate.

The body tissue 16 is represented as a capacitance 40 of the seriesresonant circuit and a resistor 42. The resistor 42 represents theresistive losses in the body tissue 16 and is a first orderrepresentation of all body tissue losses lumped together and lowers theQ of the circuit.

The series resonant circuit has a ground 44 to which the returnelectrode 18 is connected.

Finally, the series resonant circuit is completed by the transceivercircuitry 22 and the impedance matching network 24. The impedancematching network 24 is represented as an inductance in the seriesresonant circuit.

It is therefore an advantage of this embodiment of the invention that animplant 10 and method are provided which uses the body tissue 16 as anextension of the antenna 14 of the implant resulting in higher radiatedradio energy, reduced power requirements, longer life of the battery 30,an increased operating range and no dependence on the antenna 14 beingclose to a surface of the skin of the patient. In addition, because ofthe capacitive coupling between the antenna plate 14 and the body tissue16, the orientation of the implant 10 is less critical resulting in agreater choice of implant locations. In addition, the implant 10 can bemounted deeper within the body tissue 16 without significant signalloss.

Also, the size of the antenna plate 14 can be reduced resulting in asmaller implant 10 overall.

The capacitive coupling of the antenna plate 14 to the body tissueresults in higher efficiency than a metal patch on top of a titaniumshell or housing as the ground return is close to the antenna. There ismore radiated radio energy through the body tissue resulting in reducedpower requirements, increased operating range, etc.

A further major advantage of this embodiment of the invention is that,because the antenna plate 14 is preferably arranged within the housing12, the antenna plate 14 need not be of a biocompatible material. Thus,a lower cost metal can be use which is also more efficient for signaltransmission than biocompatible metals.

In addition, no feedthrough to the antenna plate 14 is required allowingfor easier manufacturing of the housing 12 without the need foradditional sealing arrangements thereby increasing the reliability ofthe implant 10. As indicated above, the antenna plate 14 can be part ofthe printed circuit board 20, thereby further facilitating manufactureof the implant 10 and obviating the need for mechanical connectionbetween the antenna plate 14 and the other circuitry.

A further advantage of this embodiment of the invention is that testingon the antenna plate 14 and the transceiver circuitry 22 can be carriedout before they are placed in the housing 12 which is also beneficialfrom a manufacturing point of view.

Importantly, the characteristic impedance of the series resonant circuitis relatively constant because the antenna plate 14 is not in contactwith the tissue thereby providing for improved power transfer.

In this regard, a simple impedance matching network 24 may be usedbeing, typically, a series inductor. This allows fewer components in theimplant 10 thereby reducing its size and resulting in lower losses inthe impedance matching circuit 24. In contrast, where a patch-typeantenna is arranged on the outside of a titanium shell, the capacitanceis very high requiring larger lossy inductors and capacitors in theimpedance matching network.

The antenna resonance is set up in the body tissue 16 as part of theantenna load and is therefore first generated and then coupled to thebody tissue. This results in increased radio efficiency.

The same antenna plate 14 can be optimised for use at multiplefrequencies when communicating on different radio bands. The antennaplate size and shape is not important to optimise radiated power. Theonly change required is to the impedance matching network 24. Thisresults in a reduced size and weight of the implant 10 and reduces thenumber of antennas 14 required for various applications. It also keepsthe antenna design efficient for designs involving multiplecommunication frequencies and bands.

Another embodiment of the present invention is shown in FIG. 4. As inthe previously described embodiment, the implant 110 includes a housing112 to be implanted in tissue, represented generally at 114, of apatient's body. The implant 110 includes a plurality of electrodes 116,118 and 120. It will be appreciated that, depending on the applicationof the implant 110, a larger number or a fewer number of electrodes 116,18 and 120 could be provided.

The implant 110 includes control circuitry 122 arranged within thehousing 112.

The electrode 116 is connected via an electrode lead 124 and through oneof a plurality of feedthroughs 126 to the control circuitry 122. Eachelectrode 18 is connected via an electrode lead 128 to the controlcircuitry 122. Similarly, each electrode 120 is connected via anelectrode lead 130 to the control circuitry 122. As is the case with theelectrode lead 124, each of the electrode leads 128 and 130 is connectedto the control circuitry 122 via one of the feed throughs 126.

The implant 110 includes a reference electrode plate 132 arranged on anoutside of the housing 112.

The control circuitry 122 includes transceiver circuitry 134. Thetransceiver circuitry 134 generates radio frequency signals to betransmitted to an external unit (not show) which communicates with theimplant 110. It also receives radio frequency signals from the externalunit. In addition, the transceiver circuitry 134 includes an impedancematching network for matching the electrode 116 and its lead 124 whenoperating as an antenna, as will be described in greater detail below.

The control circuitry 122 further includes a stimulation circuit module136. The module 136 contains electronics for effecting stimulation ofthe patient's body via the electrodes 116, 118 and 120. The module 136may be microprocessor controlled and also monitors the condition of abattery (not shown) by means of which the implant 110 is powered.Optionally, the module 136 includes measurement circuitry, other radiotransceivers and related circuitry.

The electrode 116 and its associated lead 124 can operate either as astimulation electrode for effecting stimulation at the site of thepatient's body at which the electrode 116 is located. Additionally, theelectrode 116 with its associated lead 124 can also function as anantenna for effecting communication between the transceiver unit 134 ofthe implant 110 and the external unit. The electrode 116 and its lead124 are therefore connected to the control circuitry via afrequency-dependent isolating module 138. The isolating module 138comprises a radio frequency choke 140 via which the electrode lead 124is connected to the stimulation circuit module 136 of the controlcircuitry 122. The choke 140 passes the low frequency stimulation signalwhen the electrode 116 is operating as a stimulation electrode. Thechoke 140 blocks the higher, radio frequency signal.

The module 138 further includes a radio frequency coupling element 142via which the electrode lead 124 is connected to the transceivercircuitry 134. The element 142 shows a high impedance to, and thereforeblocks, the low frequency stimulation signal to the electrode 116 butcouples, by presenting a low impedance, the radio frequency signal toground 144.

In this example, it is assumed that the electrodes 118 serve only asstimulation electrodes. Each lead 128 of the electrodes 118 is thereforeconnected to the stimulation circuit module 136 of the control circuitry122 via a radio frequency choke 146 which functions in the same manneras the choke 140.

When the electrode 116 and its lead 124 are being used as an antenna,additional electrodes 120 are used as an RF ground reference. The lead130 from each electrode 120 is therefore connected to the stimulationcircuit 136 of the control circuitry 122 via a radio frequency choke148. Each of these chokes 148 functions in the same manner as the chokes140 and 146. A ground, radio frequency short 150 connects each lead 130to ground 144 at radio frequency. The short 150 presents a highimpedance to the low frequency stimulation signal and therefore blocksthe low frequency stimulation signal while presenting a low impedancefor the radio frequency signal to couple the radio frequency signal toground.

An optional ground radio frequency short 152 and an optional radiofrequency choke 154 are also provided which perform the same functionsas the other shorts and chokes as described above.

In use, when it is desired to transmit data from the implant to theexternal unit, the electrode 116 with its lead 124 functions as anantenna with the electrodes 120 and their leads 130 functioning as aground reference for the antenna. The radio frequency signal is passedby the element 142 while any stimulation signal from the stimulationcircuit module 136 is blocked by the choke 140. Similarly, when a radiofrequency signal is received by the implant 110, the electrode 116, withits lead 124, functions as an antenna with the radio frequency signalbeing passed by the element 142 to the transceiver circuit 134.

If desired, the electrode 116 could be capacitively coupled to the bodytissue 114 of the patient so that the body tissue 114 functions as anextension of the antenna in the manner as described above. Instead, theelectrode 116 could be isolated from the body tissue 114 by placing aninductor choke (not shown) along the length of the lead 124 at theappropriate location. The choke may be manufactured into the lead 124 bycreating a coil in the wire as shown by reference numeral 151.

It is also to be noted that, instead of having the electrode plate 132arranged externally of the housing 112, it could be arranged within thehousing being coupled capacitively through the wall of the housing 112to the body tissue 114.

It is also to be noted that, if one of the electrodes is to be used as adedicated antenna, the need for the choke 140 is obviated.

While the implant 110 is intended to operate in the MICS frequency band,the implant 110 could be optimised for use at multiple frequenciescommunicating on different radio bands. The only change that is requiredis to the matching network of the transceiver circuitry 134.

The lead 124 can be made longer than the electrode requirements alone.This allows the lead to be placed close to the surface of the patient'sskin with the implant 110 in a more optimum location and to providebetter transmission signal levels.

It is an advantage of this embodiment of the invention that an existingelectrode of the implant 110, with its associated lead, is used as theimplant antenna. Optimisation of the length of the lead can result ingreater radiated radio energy, reduced power requirements, longerbattery life, increased operating range and no dependence on the antennabeing located near the surface of the patient's skin. In addition,orientation of the implant is less critical resulting in a greaterchoice of implant locations. Still further, the implant 110 can bemounted deeper in the body tissue without significant signal loss.

Because the antenna is realised by way of the electrode 116 and its lead124, the implant 110 does not have to be sufficiently large toincorporate a typical antenna. Thus, the physical size of the housing112 of the implant 110 can be reduced which simplifies implantation andresults in reduced patient discomfort.

Because one of the existing electrode leads 124 is being used as theantenna, no additional feedthroughs are required in the housing 112thereby reducing the cost of providing for hermitic sealing of thehousing 112 and reducing the cost of manufacture in general.

If the feedthroughs 126 are in the form of connectors, the desiredlength of antenna can be selected at surgery when the electrodes areplaced. This allows tailoring of the antenna for the application of theimplant 110 and further facilitates manufacture and reducesmanufacturing costs.

Common techniques for creating small antennas require the use of largeamounts of dielectric material which is usually heavy. This requirementis obviated by the present which results in a reduction in the weight ofthe housing 112 of the implant 110 and provides a greater choice ofimplant locations.

The same antenna design can be optimised for use with multiplefrequencies to facilitate communication over different radio frequencybands. This results in further reduction in size of the implant, reducesthe need for a number of antennas, reduces the weight of the implant110, is less invasive for the patient and keeps the antenna designefficient for designs involving multiple communications frequency bands.

In each of the above described embodiments of the invention, thetransmission of signals between the implanted device and an externalunit is facilitated by adapting a part of the device and/or thesurrounding tissue to function as part of the antenna means. Such anadaptive system enables the size and construction of implantable medicaldevices to be readily optimised, thereby providing more access tomedical devices for treatment of a variety of medical conditions.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

1. A medical implant comprising: a housing; a reference electrode platemounted on the housing and configured to be in direct contact with abody tissue, control circuitry contained within the housing; a pluralityof electrodes each of which is connected via a lead to the controlcircuitry; and a frequency-dependent isolating module interposed betweenthe lead of a selected electrode of the plurality of electrodes and thecontrol circuitry, the frequency-dependent isolating module for blockingstimulation signals generated by control circuitry to permit theselected electrode and its lead, reference electrode plate, and bodytissue to form a resonant circuit to function as an antenna fortransmission of signals between the control circuitry and an externalunit arranged externally of a patient's body.
 2. A medical implantaccording to claim 1, wherein each lead is fed through the housing ofthe implant through a hermetically sealed feedthrough element.
 3. Amedical implant according to claim 1, wherein the control circuitryincludes transceiver circuitry which feeds data, at a radio frequency,to the external unit.
 4. A medical implant according to claim 3, whereinthe transceiver circuitry includes an impedance matching network for theelectrode and lead functioning as the antenna.
 5. A medical implantaccording to claim 1, wherein the control circuitry includes transceivercircuitry which receives radio signals from the external unit.
 6. Amedical implant according to claim 1, wherein the control circuitryfurther includes a stimulation circuit module which contains theelectronics for generating the stimulation signals for application tothe patient's body via the electrodes.
 7. A medical implant according toclaim 6, wherein the stimulation circuit module is microprocessorcontrolled and includes a battery for operation of the implant.
 8. Amedical implant according to claim 1, wherein the frequency-dependantisolating module comprises, a radio frequency choke which passes thestimulation signals and blocks higher frequency radio signals to enablethe selected electrode to operate as a stimulation electrode.
 9. Amedical implant according to claim 1, wherein additional electrodes ofthe plurality of electrodes and their associated leads are used asground references for the antenna and for stimulation purposes.
 10. Amedical implant according to claim 9, wherein the additional electrodesare coupled to the control circuitry via isolating modules.
 11. Amedical implant according to claim 1, wherein the selected electrode isradio frequency isolated from the body tissue by providing a chokein-line with the selected electrode at an appropriate length.